class_docstr: Smooth L1 loss
  Args:
      sigma (float): hyper param in smooth l1 loss
code: def __call__(self, x, y, inside_weight=None, outside_weight=None):
      return fluid.layers.smooth_l1(
          x,
          y,
          inside_weight=inside_weight,
          outside_weight=outside_weight,
          sigma=self.sigma,
      )
  
imports: from __future__ import absolute_import
  from __future__ import division
  from __future__ import print_function
  from collections import OrderedDict
  from paddle import fluid
  from paddle.fluid.param_attr import ParamAttr
  from paddle.fluid.initializer import Normal, Xavier
  from paddle.fluid.regularizer import L2Decay
  from paddle.fluid.initializer import MSRA
label_desc: Use image node_id 2 for calling the SmoothL1Loss obj's underlying member method code with example usage: obj.__call__(x, y, inside_weight, outside_weight) and returns: fluid